The first thing to say is that it may be meaningless to ask about the
metabolism of dinosaurs in general, when the Dinosauria are so varied.
Remember that we're talking about a group that varies in mass by a
factor of forty thousand (2.5kg for Compsognathus longipes and
100 tonnes for Argentinosaurus huinculensis.)

In the same way, although we conveniently think of mammals and birds
as warm-blooded and reptiles and fish as cold-blooded, the truth - as
is so often the case - is more complex.

Some fish, including the tuna, are mildly warm-blooded, in as much as
they maintain the body temperature consistently higher than that of
the water in which the swim. Similarly, pythons may maintain a high
body temperature for months at a time, when incubating their eggs, and
some large turtles also maintain body temperature above that of the
water in which they swim.

Conversely, some mammals can be thought of cold-blooded. For example,
the smaller bats are unable to maintain their temperatures overnight
while roosting, and may approach ambient temperature. Other examples
include hedgehogs, bears, and indeed all mammals which hibernate - a
long-term lowering of body temperature.

Before looking closely at the evidence on dinosaur metabolism, we need
to define our terms.

There are several closely related issues here.
Ultimately, we are interested in whether dinosaurs had active
lifestyles like those of mammals (characterised by constant
activity) or like those of reptiles (with long periods of rest
and basking punctuated by bursts of activity.)

This lifestyle is made possible by two factors: the ability to
metabolise oxygen efficiently; and a constant, high body
temperature, at which the chemical reactions which drive the
body can proceed rapidly.

Are these two factors related?

Hence, we are interested in:

Whether dinosaurs could metabolise oxygen efficiently

Whether dinosaurs had a constant, high body temperature.

At this point the debate tends to get a bit blurred, because (with a
few exceptions) all existing animals with constant, high body
temperature achieve it via the thermal strategy in which the body's
temperature is actively maintained at a high level by a high metabolic
rate, aided by insulation in the form of fur (for mammals) or feathers
(for birds). This strategy is called endothermy - that is,
``heat from within''.

In pretty much all other existing animals, the alternative to
endothermy is ectothermy, meaning ``heat from without''.
Ectothermic animals are unable to warn their own bodies, but must
arrange for them to be warmed by some outside agency, which is why
lizards spend so much time basking on rocks, especially in the
morning: they are raising the temperature of bodies which have become
cold during the night, and this must be done before any energetic
activity, such as hunting for food, is undertaken.

The fairly clear endotherm/ectotherm division among contemporary
animals tends to blind people to the fact that there are other
possible strategies for maintaining high body temperatures.

For example, one alternative body temperature maintenance strategy -
unfortunately one which leaves no trace in the fossil record - is
controlling the rate of fermentation of food in the gut. For animals
with large guts, the amount of heat produced in this way could be
significant and the fact that no animal alive today controls this rate
as a means of modulating body temperature should not lead us to the
conclusion that it's impossible. In our haste to think of dinosaurs
by analogy with mammals, reptiles, or birds, it's sometimes easy to
forget that they were entirely different animals from anything we have
with us today. There's no reason they couldn't have regulated
temperature in an entirely different way.

Some calculations on gut-fermentation heat would be useful here.

Am I right that no existing animal uses this method of
thermo-regulation? Does anyone have a counter-example?

Is there a technical term for this thermo-regulatory strategy?

Another alternative to endothermy and ectothemy is for large animals
to maintain a close-to-constant body temperature simply by means of
their bodies' tendency to retain heat, due to their high
volume-to-surface-area ratio. (In animals of the same shape, volume
varies with the cube of the length, while surface area varies only
with the square of the length: so an animal twice as long as another
has twice as high a volume-to-surface-area ratio as its smaller
cousin.)

This strategy is called inertial homeothermy, meaning roughly
``the tendency of mass to retain the same temperature''. The large,
warm-blooded turtles mentioned above achieve their high body
temperature by this means - smaller turtles can't do it.
(Interestingly, large crocodiles, which weigh more than the largest
turtles, do not appear to be inertially homeothermic - presumably
because a croc's longer, thinner body has more surface area than the
compact body of an equivally heavy turtle.)

Am I right about the crocodiles?

Our problem is that, with all the dinosaurs having been dead for
sixty-five million years (yes, OK, except the birds, if you must), we
can't make direct measurements of any of the variables that would help
us to resolve the question of dinosaur body temperatures. If we could
measure, for example, the rate at which dinosaurs consume oxygen when
at rest, we could determine their base metabolic rate; or if we could
get a thermometer into the appropriate part of the anatomy - a risky
business - we could measure the internal temperature directly. In the
absence of such data, we must reason from more indirect evidence.

It's been noted that the two existing classes of endothermic animals -
the mammals and birds - are also the two only classes that walk erect
(that is, with the legs underneath the body rather than sprawled out
sideways as in reptiles and amphibians.) Crocodiles have a
semi-erect, semi-sprawled posture, and chameleons have a close to
erect posture, but are not quite there; both cases are very much the
exception to the general rule among reptiles.

Since dinosaurs also had erect posture - their skeletons tells us this
unambiguously - it doesn't seem unreasonable to assume that they too
were endotherms.

But why should this be? In the 60s, John Ostrom postulated that
endothermy (or at least high, constant body temperature, however
achieved) is actually a pre-requisite for the erect posture, as erect
animals must perpetually support their own weight, whereas sprawlers
spend most of the time resting their bodies on the ground between
their feet. The best part of forty years on, this connection has yet
to be proved, but it is at least suggestive.

We can go further by looking at dinosaur skeletal features beyond the
simple erect posture. Many groups of dinosaurs, including the
ornithomimids, dromaeosaurs, troodontids and even tyrannosaurs are
clearly built as cursorial animals (that is, with special adaptations
for running.) Skeletal features are shared with fast-running
flightless birds and even fast quadroped mammals such as horses,
cheetahs and antelopes: these include the enlongation of the tibia
relative to the femur, the enlongation of the toes, etc.

What are the other cursorial adaptations?

It seems nonsensical that these adaptations should have arisen in
animals that lacked the metabolism to make us of them for sustained
running, so this seems to give us more evidence that at least these
groups of dinosaurs had high metabolism.

What exactly is the relation between metabolism and body temperature?

A final related piece of evidence comes from fossilised dinosaur
tracks. According to McNeill Alexander's Dynamics of Dinosaurs and
Other Exinct Giants, there theropod tracks in Texas which indicate
running speeds on the order of 12 meters per second (about 27 miles
per hour.) This compares favourably with the best human sprinters (10
m/s) and respectably with zebras, giraffes and various antelopes as
measured in the wild (11 to 14 m/s), although it's not as fast as
specially bred race-horses (17 m/s) and greyhounds (16 m/s).
Nevertheless, an animal that runs as fast as an antelope might be
expected to have a metabolism able to sustain that pace for long
enough to be worthwhile.

(The value of the evidence from fossil trackways is made less clear by
the confusion that seems to exist concerning reptilian stamina levels.
Some sources imply that reptiles can sustain fast movement for only a
matter of seconds: development of cursorial adaptation that could be
used only for such short durations would be surprising. On the other
hand, there are other sources which describe crocodiles strenuously
resisting capture for up to half and hour before becoming exhausted:
An ectothermic cursorial dinosaur seems a reasonable proposition if it
could run for that long before needing to stop and rest.)

If we accept, as most palaeontologists now do, that birds are
descended from coelurosuarian dinosaurs, then we have to consider at
what stage in the evolution of birds endothermy appeared. It must
surely have been in place for the first flyers, since by universal
consent, a high metabolic rate is absolutely necessary in order to
sustain powered flight. So if Archaeopteryx was endothermic,
why should not its immediate ancestors have been?

This line of reasoning has been considerably strengthened in recent
years with the discovery of feathered skin impressions from five
separate dinosaurs (although, slightly suspiciously, all from the same
locality in China):

Sinosauropteryx, a compsognathid, in 1996

Protarchaeopteryx, a maniraptor of some kind, in 1997

Caudipteryx, a probable oviraptorosaur, in 1998

Beipiaosaurus, a primitive therizinosaur, in 1999

Sinornithoaurus, a primitive deinonychosaur, in 1999

The extraordinary thing about these finds is that they encompass a
huge part of the coelurosaurian group, so that it's reasonable to
suppose that all maniraptoriformes had feathered, or at least, if not,
they had discarded them. In fact, the only known coelurosaurian skin
impression without feathers is from the tail of what may be a
tyrannosaur; it's easy to imagine that tyrannosaurs and other very
large coelurosaurs may have lost their feathers in much the same way
as elephants, rhinos and hippos have lost their fur.

The significance of feathers is that they make sense only for
endothermic animals, as they are an insulation. For animals which
generate their own heat internally, feathers help to prevent that heat
radiating away; but for ectotherms, feathers (like fur) would only
hinder the basking process. So we can conclude that all feathered
dinosaurs were endothermic. (Or at least that they maintained a high,
constant body temperature, perhaps by inertial homeothermy in the
larger species - but while that may be an option for
Beipiaosaurus, it would have been impossible in smaller animals
such as Caudipteryx.)

Most experts believe that dinosaurs and pterosaurs evolved from a
common ornithodiran ancestor. If we accept that pterosaurs must have
been endothermic in order to sustain powered flight (and the smaller
species could not have been gliders, even if the larger ones were),
then we may consider that the common ancestor was likely also to have
been endothermic, so that the most primitive dinosaurs shared this
property.

It's tempting to go further, and say that therefore all dinosaurs must
have been endothermic, but it doesn't necessarily follow: there is
some evidence that certain prehistoric crocodiles were endotherms, but
that condition has ben lost in modern crocodiles, perhaps because of
their aquatic lifestyles. It's possible, then, that even if primitive
dinosaurs were endothermic, their descendents may not have been.

The pterosaur line of argument is not a strong one, having about it a
strong whiff of ``if this, then that''. Nevertheless, it is one more
shovelful of evidence to fling onto the endothermy bandwagon.

###
fibro-lamellar (dense primary) => fast growth => high metabolism
but some bones show growth rings (rare but not unknown in mammals/birds)
haversian bone (rich in blood vessels) (but also in large ectotherms
such as crocodiles and turtles; absent in some small endotherms)

Stop press: literally as I write this (28th November 2000), I read
news of a newly-published paper in which ### compare growth-rings in
three theropod species and in contemporary crocodilians, observing
that the rings are more pronounced in the crocs. From this, they
conclude that the dinosaurs had more stable metabolism than crocodiles
(though this doesn't tell us anything about how how they compare with
contemporary mammals - surely a fruitful follow-up study for someone!)

A six-tonne african elephant eats 250kg of food per day (1/24th of its
body mass) and spends thirteen hours of every twenty-four gathering
it, despite its huge head.) Lions need to eat their body mass every
nine days, wild dogs every seven days, and Komodo dragons only every
ninety days.

Just kidding. Of course I'll give you a conclusion. The important
thing to realise is that any conclusion in a debate as wide-ranging as
this one, taking into account so many factors, so many of which are
still ill-understood, must necessarily pass out of the domain of fact
into that of opinions.

With that disclaimer out of the way, my feeling is that dinosaur
skeletons tell us enough for us to be pretty sure that they led active
lifestyles, which entails a high, more or less constant, body
temperature. Whether they achieved this temperature by means of
mammal-like endothermy or inertial homeothermy seems to me a very much
less important question. The answer is probably ``both and neither''
- no doubt dinosaur metabolism made use of their mass to maintain
stability of body temperature; but for smaller dinosaurs, and
especially those which gave rise to the birds, it seems perverse to
argue that they didn't also maintain body temperature internally.

Informed opinion is that the smaller theropods and ornithopods were
the dinosaurs most likely to be endothermic in the manner of modern
mammals and birds. It happens that these kinds of dinosaur also have
the largest relative brain size. It's interesting to speculate on
whether there's a causal link between these two properties, or whether
it's just a coincidence (but I'm not going to do it here.)

In the end, there's no reason to assume that animals which in so many
ways are so different from anything alive today had a metabolic
strategy exactly like any of the extant groups.